1. Field of the Invention
The present invention relates to a method of making axial alignment of a charged particle beam and also to a charged particle beam system.
2. Description of Related Art
In recent years, charged particle beam systems including scanning electron microscopes for observing and measuring microscopic structures such as living organisms, materials, and semiconductors and metrology SEMs for measurements of circuit patterns of semiconductor devices have been known.
FIG. 31 is a block diagram showing one related art scanning electron microscope. The microscope, generally indicated by reference numeral 1000, has an electron gun 102 producing a charged particle beam B that is passed through an objective aperture 103. Then, the beam is focused on a surface of a sample S by an objective lens 106. When the beam B is scanned over the sample S by scan coils 107, secondary electrons and backscattered electrons are emitted from the surface of the sample S. These electrons are received by an electron detector 108. The intensity is displayed on an image display device 109 in synchronism with the beam scanning.
Axial alignment of the charged particle beam B with the objective aperture 103, an aperture angle correcting lens 105, and the objective lens 106 of the scanning electron microscope 1000 is made by controlling currents flowing through a beam axis alignment coil (hereinafter referred to as the X alignment coil) 104a and another beam axis alignment coil (hereinafter referred to as the Y-alignment coil) 104b which deflect the beam in the X-axis and Y-axis directions, respectively. The beam B can be deflected in two dimensions by combination of the alignment coils 104a and 104b. It is assumed here that the optical axis of the objective lens 106 lies in the Z-direction.
In particular, axial alignment of the charged particle beam B is made as follows. First, a signal is sent from a signal generator 110 for a wobbler to a high-voltage source 111 for producing an accelerating voltage to vary the accelerating voltage of the beam B in equal, small increments such that the focal state of the beam B on the surface of the sample S is varied minutely. If the optical axis deviates, the beam B crosses the objective lens 106 obliquely. Therefore, during execution of the wobbling motion, as shown in FIG. 32A, the image I is moved in a given direction when the focal state is being varied. On the other hand, if the optical axis is aligned, the beam B passes through the objective lens 106 vertically and, therefore, during the execution of wobbling motion, as shown in FIG. 32B, the image I hardly moves in spite of variation of the focal state. Accordingly, the values of the currents flowing through the alignment coils 104a and 104b can be adjusted to align the optical axis by manually adjusting XY adjustment knobs 112 to minimize the amount of motion of the image I while observing the amount of motion of the image I during the execution of wobbling motion. The adjustment knobs 112 include an X adjustment knob for adjusting the value of the current in the X alignment coil 104a and a Y adjustment knob for adjusting the value of the current in the Y alignment coil 104b. The values of the currents in the X alignment coil 104a and Y alignment coil 104b can be controlled via a driver amplifier 113 by manipulating the XY adjustment knobs 112. A wobbling motion can also be made by varying the exciting current in the objective lens 106, the current being supplied from an objective lens driver amplifier 115.
Methods of automating this axial alignment are disclosed, for example, in JP-A-7-302564, where an adjustment method of varying the axis in increments to obtain an optimum value for axial alignment and a method of determining the direction of an adjustment made to the axis according to the direction of image displacement in a case where the objective lens excitation is varied are presented.
In the above-described scanning electron microscope 1000 shown in FIG. 31, the resultant vector of the direction of axial deviation of the charged particle beam, the direction of deflection achieved by the X alignment coil 104a, and the direction of deflection achieved by the Y alignment coil 104b is intrinsically a vector representing the motion of the image at the wobbler. Therefore, if one of the X alignment coil 104a and Y alignment coil 104b is adjusted optimally, the image motion induced by the wobbler remains. Accordingly, if the X alignment coil 104a is adjusted first and inappropriately, it is impossible to find a point at which image motion does not occur however the Y alignment coil 104b is adjusted. In this way, the adjustment operation for searching for one point where no image motion occurs by adjusting the two alignment coils is not intuitively understandable for the operator of the instrument and places great burden on the operator.
In view of this problem, JP-A-7-302564 discloses an apparatus that automates the axial alignment. In the axial alignment method of this patent document, however, an operation needs to be performed repeatedly many times to find an optimum value for axial alignment.